Individual infant rats are notoriously inept at basic regulatory functions, yet in clumps or huddles, pups display "group regulatory behavior." This proposal aims to achieve a deeper understanding of the ontogeny ofhuddling as contact behavior which will become a generalizable and integrative approach in psychobiology, with potentially profound implications for the health and development of human infants. To this end, a model was developed of huddling by infant rat pups in which complex group dynamics emerge from individuals following simple rules: the model generates patterns of contact behavior that resemble those observed in real pups. Using this model, tests are proposed that will challenge it with a variety of perturbations: (1) different ambient temperatures, (2) cycles of ambient temperature, (3) variations in number of interacting pups, and (4) variations in age (maturity) of pups. In each case, identical behavioral parameters in the model and in rat pups will be measured. Individual and group behavior will be compared quantitatively. Dimensions of practical and theoretical importance, e.g., huddling configurations, activity states, stability, and developmental trajectories can be used to test the model, as well as to diagnose the organization of group behavior through development. A second set oa Aims--designed in anticipation of improving the model-are also proposed. Swarm (an advanced computer modeling system developed at the Sante Fe Institute on Complexity) will be used to (5) model huddling and movement patterns in complex environments, as well as (6) expand athe model into a form that defines, describes, and incorporates ontogenetic changes involving sensory and motor capacities.